The present invention relates to a sports tool using tungsten and a method of manufacturing the same. The invention maybe applied to for golf things, e.g., golf club heads, fishing gears, e.g., lure, reel, chum cage, and sinker, and parts of a bicycle. The golf club head is classified into a golf club head of a hollow metal shell type, also called a wood type golf club, a putter, and an iron golf club. For the reel, a bail holder may be enumerated.
Of those golf clubs, the putter and iron clubs, by convention, are made of a metallic material of an iron or titanium family. When two golf club heads of the same size are compared, one golf club head having a larger moment of inertia about the center of gravity than the other is broader in sweet area. Accordingly, even when the impact point is out of the sweet spot, the golf club head swung is stable, and a direction of a hit ball is stable. In this respect, the golf club head having the large moment of inertia is preferable. For this reason, it is desired that the golf club head is made of a material having a large specific gravity. Examples of metals each having a large specific gravity than iron or titanium are copper, lead and tungsten. Examples of things which are desirably made of materials each having a large specific gravity are a sinker, and a lure in the light of size reduction possibility. Those things may be applied to parts of a bicycle.
Copper and lead, however, are limited in their applications to sports goods since those are too soft. Tungsten has the largest specific gravity in the metals mentioned above. If tungsten can be used for the golf club head, the following advantages are gained: A moment of inertia of the golf club head is increased and the size of the golf club head is reduced for the same weight. The golf club head using tungsten is disclosed in Japanese Patent Application Laid-Open Publications (JP-A) Nos. 07-216490 and 09-262326. The former golf club head is manufactured by sintering, and the latter one, by sintering and forging.
A melting temperature of tungsten is high, 3370° C. To mold industrial goods made of tungsten, it is impossible to melt and forge the metal material. Accordingly, usually, metal powder is sinter molded as disclosed in the publications. The sinter molding has industrial problems, however. It is difficult to mold the product in a complicated shape. A sintering time is generally long. A sintering temperature is still high, 1500° C. A dedicated mold is needed. Cost of the molding die and cost to mold are not inexpensive. In carrying out the forging process following the sintering process, the problems on the sintering process exist, and further a step for the forging is needed, and much and troublesome labor is required. A molded product by the sintering process has a high hardness. It is difficult to work the molded product by machining and polishing, and hence the finishing work is difficult.
In order to, for example, stabilize the impact by the golf club, the position of the center of gravity of the club head has been adjusted. Club structures each with a weight part or a weight piece are disclosed in some patent publications.
Japanese Patent No. 2526530 discloses a golf club head in which a window hole is formed in a rear part of the head hollow part, and a weight having a larger specific gravity than the head body is caulked at the window hole.
Japanese Patent Unexamined Publication NO. 2001-129134 discloses a golf club head in which an opening is formed in a bottom part of a hollow shell head body made of maraging steel, a sole piece provided with a partially thick weight part made of stainless steel is welded at the opening.
Japanese Patent Unexamined Publication No. Hei.10-94623 discloses a golf club head in which the weight piece with a through hole is made of a tungsten sintered alloy of 10 or higher in specific gravity, and is held by passing a pin through the through hole and deforming the pin.
Japanese Patent Unexamined Publication No. Hei. 10-201887 discloses a golf club head in which a weight piece made of tungsten, for example, is secured with a cladding welding part.
In JP-A-2001-129134, since iron family metals (steels) are jointed together by welding so as to secure an easy welding, a specific gravity of the weight part is a little different from that of other parts of the club head. As a result, the effect by the weigh part is small. When the weight is fastened by caulking, the caulked part is loosened with its use, and will generate abnormal noisy sound when the ball is hit with the club head. In the case of JP-A-Hei. 10-94623, the weight piece has a large specific gravity. Accordingly, the weight piece efficiently exhibits its weight function. This structure will suffer from the loosening as in JP-B-2526530, however. Further, the pin is exposed to outside, making the outward appearance unattractive. In JP-A-Hei. 10-201887, when the weight piece is made of (pure) tungsten. The cladding welding member freely varies its shape in its welding state. Accordingly, the weight piece is secured in a state that it is held down from above. It is difficult to melt the weight piece itself, and no technical disclosure of the welding condition in the sense of melting the weight piece itself is presented in the patent publication. What is disclosed is only the fact that the cladding welding part is used as a brazing filler metal. The joint strength by the brazing is weaker than the strength of the welding. Accordingly, the brazing is unsatisfactory in securing a reliable durability of the golf club head.